1. Field of the Invention
This invention relates to a paper feed tractor for supplying paper having feed holes formed along the opposite longitudinal edges therein to a printer for use with a computer, a word processor, a plotter and so forth or to a printer of various other types.
2. Description of the Prior Art
Paper having a pair of rows of feed holes or perforations formed in an equidistantly spaced relationship therein along the opposite longitudinal edges thereof is used very frequently on a printer for use with a computer, a word processor, a plotter or the like as paper on which characters or figures outputted from the printer are to be recorded. A paper feeding device for supplying paper of the type mentioned to a printer normally includes a pair of paper feed tractors disposed at locations adjacent the opposite longitudinal edges of paper to be fed by the paper feeding device. An exemplary one of such paper feed tractors which are widely known in the art includes a sprocket wheel supported for rotation on a tractor frame, a belt guide formed on the tractor frame, and an endless belt extending between the sprocket wheel and the belt guide and adapted to be driven by the sprocket wheel to travel along a guide path formed by the belt guide. The endless belt has feed pins provided projectingly in a row on an outer peripheral face thereof at the same equal intervals with feed holes formed in a row along a longitudinal edge of paper of the type described above.
Further, a paper feed tractor of the type just mentioned normally has such a construction that a semi-arcuate guide portion having a substantially same diameter as the outer diameter of the sprocket wheel are formed at a location of the belt guide remote from the sprocket wheel while a pair of linear guide portions are formed in a contiguous relationship to the semi-circular guide portion in directions tangential to an outer periphery of the sprocket wheel such that the endless belt may travel along arcuate guide paths on the sprocket wheel and the semi-arcuate guide portion and linear guide paths contiguous in a circumscribing relationship to the arcuate guide paths, and paper is fed by the endless belt while the endless belt travels along one of the linear guide paths.
When a character or a figure is to be printed on paper on the printer, normally a high speed paper feeding motion is transmitted to the endless belt and such motion and stopping are transmitted to the endless belt intermittently and repetitively. On the one linear guide path, the endless belt feeds the paper by way of the feed pins provided projectingly on the outer peripheral face thereof two or more of which pins are simultaneously engaged with the feed holes formed along the corresponding longitudinal edge of the paper. The feed pins provided projectingly on the endless belt are each formed, at an end portion thereof, in a truncated conical shape in order to facilitate insertion of the feed pin into a feed hole formed in paper. Further, in order to facilitate release of each feed pin from a feed hole, the outer diameter of a portion of each feed pin at which the feed pin engages with a feed hole of paper is made a little smaller than the diameter of the feed hole. Due to such difference in diameter between the feed holes and the feed pins, if the endless belt is stopped abruptly while the paper is moving at the same speed together with the endless belt, the paper will be stopped, as a result of continued movement due to inertia of the mass thereof, at a position advanced by a distance corresponding to the difference in diameter from the stopped position of the endless belt. Then, if the endless belt is subsequently moved suddenly, then the feed pins of the endless belt will first travel by the distance corresponding to the difference in diameter and then collide and be engaged with end edges of the feed holes of the paper to thereafter cause the paper to travel. Therefore, where the quality of the paper is low or the humidity of external air is high, the paper may be deformed or broken at portions of the feed holes thereof at which the feed pins of the endless belt are collidingly engaged with the feed holes. Further, since the feed holes are formed proximate the opposite longitudinal edges of the paper, the paper is normally acted upon by a force to tend to move the paper toward a central location of the paper perpendicularly to the direction of travel of the endless belt due to resistance applied to the paper by a printing mechanism, a platen and so forth of the printer. Such phenomenon of deformation of feed holes of paper or break of paper is promoted by the force, which makes a cause of deterioration in accuracy in paper feeding and in accuracy in printing of the printer.